feat: extension and plugin loading system

[onerandomusername]

Description

Adds the ability to load extensions, and manage them.

Most of the code of the first commit is taken from this project, at that position in time.

Added

• Bot modes to determine behaviour. Multiple can be applied at once.
  • PROD: the default mode, no dev extensions or dev plugins load
  • DEVELOP: the bot developer mode, most useful for people adding features to modmail
    • Enables the extension_manager extension.
  • PLUGIN_DEV: the plugin developer mode, useful for enabling plugin-specific features
    • This is not used yet.
• Extension loading system
  • scans for extensions in the modmail/extensions folder and loads them if they are of the right format.
• Plugin loading system
  • scans for plugins in the modmail/plugins folder and loads them.
• Extension management commands
  • load, reload, unload, list, refresh commands for dealing with extensions
  • Run the ext command for more details when the bot is in DEVELOP mode.
• Plugin management commands
  • load, reload, unload, list, refresh commands for dealing with plugins
• • Run the plugins command for more details.
• Extension metadata
  • used to determine if a cog should load or not depending on the bot mode
• Plugin helper file
  • modmail/plugin_helpers.py contains several helpers for making plugins
    • PluginCog
    • ModmailBot, imported from modmail.bot
    • ModmailLogger, imported from modmail.log
• Meta Cog
  • NOTE: The commands in this cog is not stabilized yet and should not be relied upon.
  • Prefix command for getting the set prefix. Most useful by mentioning the bot.
  • Uptime command tells the end-user how long the bot has been online.
  • Ping command to see the bot latency.
• Guide on how to contribute to modmail, see CONTRIBUTING.md
• Start a Changelog
• Add changelog style verify CI check

Fixed

• Make the bot http_session within an event loop.

Todo in later issues:

• manage installing plugins and updating them from Github
• metadata to determine when in bot start to load extensions (deferred to later pull)
• metadata to determine dependencies on any other extensions/plugins (deferred to later pull)
• test suite for all of this new code (deferred to later pull)
• mock attributes for discord py components (deferred to later pull)

[codecov]

Codecov Report

Merging #43 (f708632) into main (a65e576) will decrease coverage by 54.59%.
The diff coverage is 23.15%.

@@             Coverage Diff             @@
##             main      #43       +/-   ##
===========================================
- Coverage   91.51%   36.92%   -54.60%     
===========================================
  Files           5       12        +7     
  Lines         283      520      +237     
  Branches        7       60       +53     
===========================================
- Hits          259      192       -67     
- Misses         20      323      +303     
- Partials        4        5        +1     

Impacted Files	Coverage Δ
modmail/extensions/extension_manager.py	0.00% <0.00%> (ø)
modmail/extensions/meta.py	0.00% <0.00%> (ø)
modmail/extensions/plugin_manager.py	0.00% <0.00%> (ø)
modmail/plugin_helpers.py	0.00% <0.00%> (ø)
modmail/bot.py	43.05% <32.65%> (-21.05%)	⬇️
modmail/utils/plugins.py	39.53% <39.53%> (ø)
modmail/__main__.py	47.05% <50.00%> (-2.95%)	⬇️
modmail/utils/extensions.py	60.46% <60.46%> (ø)
modmail/utils/cogs.py	90.00% <90.00%> (ø)
modmail/__init__.py	100.00% <100.00%> (ø)
... and 9 more

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[onerandomusername]

Once @bast0006 re-reviews, I'm planning to merge this tomorrow.

Approved in discord